The human brain uses many visual cues to perceive depth, including relative size, lighting and shadows, focusing, and most importantly binocularity. Because the right and left eyes are located at different points in space, each views a scene from a slightly different perspective. This is called binocular parallax. The image of a three dimensional world is largely a construct of the mind generated by comparing and integrating these two slightly disparate two dimensional images.
Optical methods of producing three dimensional illusions by presenting slightly different images to the two eyes are called stereoscopy. One common type of stereo viewer separates the two images spatially and is a direct descendant of the postcard stereo viewer of the 19th century and its predecessor the Wheatstone mirror stereoscope. Virtual reality computer systems used today are often based on this idea and employ head mounted displays which use a pair of small video screens to present stereo pairs of images to the two eyes of the user.
Other systems have also been used to create three dimension illusions. For example, one can use a lenticular screen made of vertical cylindrical lenses and a stereo pair of images which are vertically sliced and alternated in a composite image under the screen. When this composite image is viewed at a certain distance, the right eye sees only the right image and the left eye sees only the left image. This is the basis for three dimension photography which does not require a viewer.
Stereo pairs of images can be time multiplexed so that first an image is presented to the right eye alone and then its stereo pair is presented to the left eye alone. This can be realized by mechanical means or with liquid crystal electronic shutter glasses. These glasses open first the right and then the left eye, while alternately closing the other eye, in synchrony with the changing image on a video monitor (i.e., TV) or other device.
Other methods and systems for generating a stereoscopic image include an anaglyphic system which employs complimentary colored (e.g. red and green) glasses to view a superimposed stereo pair of images which were photographed through filters of the same colors. This system, which is widely known due to its use for motion picture "3-D horror movies", is compatible with video monitors but is not compatible with a true color image. Related systems are also known which use polarized glasses to allow discrimination of right and left eye images projected through vertical and horizontal polarizers. While these systems can be used for true color images, these systems are not useable with a video monitor.
Still other methods used for creating three dimension illusions include random dot stereographs and those based on the Pulfrich effect.